A photonic network provided with a reconfigurable optical add/drop multiplexer and/or a wavelength cross connect has been proposed and developed. A reconfigurable optical add/drop multiplexer branches an optical signal of a requested wavelength from a WDM signal to guide the signal to a client, and adds a client signal of the requested wavelength into the WDM signal. A wavelength cross connect (WXC, or a photonic cross connect (PXC)) may control the route of an optical signal for each wavelength without converting an optical signal into an electric signal.
In the above-mentioned photonic network, a plurality of optical paths (wavelength paths in this case) that use the same wavelength may be established. Therefore, to configure or operate a network without fail, for example, a scheme of superimposing a path ID which identifies each optical path on an optical signal and transmits the signal has been proposed. In this case, an optical node device (a reconfigurable optical add/drop multiplexer, a wavelength cross connect, etc. in this example) in the photonic network has the function of detecting a path ID superimposed on the optical signal. In this case, since each optical path may be identified without fail in the optical node device, a fault that an optical fiber is connected to a wrong port etc. may be monitored, detected, and avoided.
An optical transmission device in an optical fiber transmission system has been proposed as one of the related technologies (for example, Japanese Laid-open Patent Publication No. 2004-70130 (U.S. Pat. No. 3,763,803)).
A signal indicating a path ID (hereafter referred to as a path ID signal) is superimposed on an optical signal in, for example, an intensity modulation format. However, in this case, a cross gain modulation occurs in an optical amplifier (for example, an EDFA) which collectively amplifies a WDM signal and/or stimulated Raman scattering in an optical fiber. The cross gain modulation may cause a crosstalk of a path ID signal between wavelength channels in the WDM signal. As a result, in an optical node device, there is the possibility that a path ID is erroneously identified.
Under the situation above, a method of superimposing a path ID signal on an optical signal in the frequency modulation format has been studied. However, when an optical signal passes through a plurality or a large number of optical nodes, the detection sensitivity of a path ID signal may drops. When the detection sensitivity of the path ID signal drops, an optical transmission device may be unable to correctly recognize the path ID of a received optical signal, thereby causing a trouble in establishing a network. The problem is not limited to the case where a path ID signal is superimposed on an optical signal, but may occur when a signal is superimposed on an optical signal in the frequency modulation format.